Hello, I have noticed many brown spots on my Sweet chestnut leaves. They keep appearing. What this could be and how should i treat this.

Move it in a bigger container or, even better, in the ground outside: chestnuts are fast growing trees and don't like to be container-bound. That's why they are usually trasplanted when still small.
What you are seeing is stress due to the roots trying to expand and finding no room and perhaps a closely tied watering issue.

Thank you for advice. I will move Chestnut in big container